Aerospace Application Frequency Comb

(Authors: Esther Baumann, Jeff Chiles, and Ian Coddington,

Affiliation: National Institute of Standards and Technology (NIST), USA)

(Authors: Andrew Attar and Kevin Knabe, Affiliation: Vescent Photonics)

(Author: David Carlson, Affiliation: Octave Photonics)

▎Abstract

Since their inception, frequency combs have opened a series of exciting new scientific applications. They not only fulfill existing priorities of NASA but also enable unique time and frequency precision, facilitating entirely new experiments in the future [1]. Future space experiments will require the development and validation of optical frequency combs compatible with harsh environments while maintaining small size, light weight, and low power consumption. Space-deployable frequency combs will have a broad impact on NASA's next-generation precision frequency and timing applications.

▎Main Text

Over the past decade, frequency combs have become a key enabling technology for precision measurement applications, providing frequency stability of 10^-19 across the optical and microwave spectrum and achieving attosecond-level time measurement capabilities. This unique ability has opened a range of extraordinary and novel sensor applications. For NASA, frequency combs have been proposed for a series of important uses, such as precise radial velocity measurements of satellites, nanometer-level distance measurements supporting ultra-precise satellite formation flying, detection of gravitational anomalies, femtosecond-level time transfer for millimeter-wave telescope array synchronization, sensitive detection of greenhouse gases, and high-fidelity dividers critical for optical atomic clocks.

Figure 1. Application of new optical frequency comb arrays in space

Although these application concepts are exciting, developing deployable frequency combs capable of such high-precision measurements in harsh space environments is a significant challenge. First, the robustness of deployable frequency combs must be increased to survive launch and ensure remote operation. Second, the size, weight, and power (SWaP) of their optical systems must be reduced to enable operation on satellites. Additionally, costs must be lowered. After all, for most of the above precision measurements, the frequency comb is only a subsystem and cannot exceed the total instrument budget. Considering SWaP and cost reduction, erbium-doped fiber frequency combs are a particularly promising choice for space-deployable frequency combs. Currently, fully polarization-maintaining erbium-doped fiber frequency combs have demonstrated good compactness and long-term reliability [10,13], supporting 10^-15/s^1/2 frequency stability and phase-slip-free operation, meeting nearly all timing and fidelity requirements [11]. Meanwhile, fiber frequency combs have been successfully launched on sounding rockets [10], and mode-locked femtosecond fiber oscillators have been demonstrated in low Earth orbit [14]. These experiments represent only the first steps toward deploying erbium fiber frequency combs in space, but much work remains to mature the technology. For example, existing radiation-hardened designs reduce SWaP capability and decrease system stability under harsh environmental conditions.

One factor driving the reduction of SWaP and enhancement of radiation resistance in these devices is the high pulsed optical power of frequency combs. Fiber amplifiers are required to achieve nonlinear frequency conversion for stable optical frequency combs. Traditionally, highly doped erbium fiber amplifiers necessarily use highly nonlinear fibers to generate high-power pulses and broaden their spectra. The pump diodes driving these fiber amplifiers may consume over 10 W of power or account for more than half of the total power consumption of the frequency comb. Additionally, because most common germanium-doped telecom fibers increase loss under solar radiation, the selection of doped fibers suitable for space frequency combs is greatly reduced compared to ground-based combs. Recent advances by NIST and Octave Photonics in fiber-coupled nonlinear waveguides [15-16] show that by confining light within custom waveguides, optical frequency conversion efficiency can be significantly improved, substantially reducing the power consumption of optical amplifiers. Besides significantly lowering SWaP, another advantage of nonlinear coupled waveguides is their good radiation resistance. Therefore, NIST, Vescent Photonics, and Octave Photonics have recently been working to commercialize this technology.

Figure 2a) Current Vescent fiber frequency comb module. Includes comb optical components, photodetection circuits, and cavity oscillator length control circuits. For thermal management reasons, pump diodes are currently not included in this package but are planned for future versions.

Figure 2b) Fiber oscillator module or the "engine" of the frequency comb system. This module contains optical elements and cavity length control actuators but excludes electronic components or pump diodes.

Figure 2c) Prototype radiation-hardened fiber frequency comb developed under Air Force BAA funding. Some of these systems are currently being evaluated for satellite deployment. The system includes initial prototypes of Octave Photonics' nonlinear waveguides.

Figure 2d) Pump diode module for frequency comb applications. This device includes current and temperature controllers and can accommodate up to four pump diodes to drive two frequency comb systems.

Over the past six years, Vescent has used SBIR (Small Business Innovation Research) and U.S. Air Force BAA (Broad Agency Announcement) grants to reduce the size, weight, power consumption, and cost of fiber frequency combs while improving their robustness against environmental impacts for space deployment. Most of this funding has come from the U.S. Air Force and NASA to advance fiber combs for next-generation positioning, navigation, and timing (PNT) applications, including optical atomic clocks and two-way time transfer. Some of this work involves designing, manufacturing, and testing radiation-hardened fiber frequency combs for satellite deployment of dual-photon rubidium clocks. Although progress has been made in reducing size and moving these systems out of the lab, more effort is needed to meet the size, power, and overall robustness requirements suitable for CubeSat operation. Continued development of radiation-hardened frequency combs and nonlinear nanophotonics can enable comb platforms supporting cutting-edge science. While progress has been made in reducing SWaP and transitioning these systems from the lab, further efforts are required in size, power consumption, and overall durability to meet CubeSat operation needs. Future advances through radiation-hardened frequency combs and nonlinear nanophotonics can realize support for such advanced optical frequency comb systems.

▎Conclusion

Although efforts to develop low SWaP and radiation-resistant optical frequency combs have achieved initial success, there is still a lot of work to be done to deploy them on satellites. Further reducing system SWaP is crucial to support CubeSat missions. Additionally, although frequency combs are ready for flight testing and have been preliminarily demonstrated in Europe and Asia, this technology has not yet flown within the United States. NASA's efforts in low SWaP and radiation resistance for optical frequency combs will bring benefits for future applications.

References

Principles and Background

Space Optical Frequency Comb Application 1: Precision Radial Velocity Measurement for Exoplanet Detection

· Principle: When an exoplanet orbits a star, it causes slight radial velocity changes in the star. By measuring the Doppler shift of absorption lines in the star's spectrum, the star's radial velocity changes can be inferred, leading to the discovery of exoplanets. Frequency combs provide a high-precision frequency reference for accurately measuring frequency changes in the star's absorption lines, thereby improving the precision of radial velocity measurements.

· Advantage: Traditional radial velocity measurement methods have limited precision and struggle to detect smaller or more distant exoplanets. The high frequency stability and precision of frequency comb technology can achieve radial velocity measurement precision at the centimeter-per-second level or better, greatly increasing the probability and accuracy of exoplanet discovery.

Space Optical Frequency Comb Application 2:

Nanometer-scale Distance Measurement to Support Ultra-Precise Satellite Formation Flying or Gravity Anomaly Detection

· Principle: Based on heterodyne dual optical frequency comb multi-wavelength interferometric ranging, the highest precision distance measurement is performed using a single comb tooth, with synthetic wavelengths at various levels extending the measurement range. By measuring the time delay of light propagation between satellites and combining it with the high-precision frequency measurement capability of the frequency comb, nanometer-level distance measurement is achieved.

· Advantage: In ultra-precise satellite formation flying, nanometer-level distance measurement precision ensures accurate control of relative positions between satellites, enabling more complex space missions such as synthetic aperture radar interferometry. For gravity anomaly detection, precise distance measurement helps monitor subtle changes in Earth's gravity field, which is important for studying Earth's internal structure and ocean circulation.

Space Optical Frequency Comb Application 3: Femtosecond Time Transfer to Synchronize Millimeter-Wave Radio Telescope Arrays

· Principle: Using the optical frequency comb as a bridge between the optical and radio frequency bands, femtosecond pulse sequences generated by the comb are transmitted via optical fiber or free space to various telescope sites as a timing synchronization reference signal. By precisely controlling the transmission time and frequency of femtosecond pulses, high-precision time synchronization among telescopes in millimeter-wave radio telescope arrays is achieved.

· Advantage: Millimeter-wave astronomical observations require multiple telescopes to work together to form an interferometric array to improve observation resolution. High-precision time synchronization is key to ensuring interferometric measurement accuracy. The femtosecond time transfer technology of frequency combs can improve time synchronization precision to the femtosecond level, greatly enhancing the observational performance of millimeter-wave radio telescope arrays.

Space Optical Frequency Comb Application 4: Sensitive Detection of Greenhouse Gases

13. Joohyung Lee, et.al., "Testing of a femtosecond pulse laser in outer space", Scientific Reports,4, 5134 (2014).

14. D. R. Carlson, et.al., "Self-referenced frequency combs using high-efficiency silicon-nitride waveguides," Opt. Lett. 42, 2314–2317 (2017).

15. Jeff Chiles, et.al., "Multifunctional integrated photonics in the mid-infrared with suspended AlGaAs on silicon", Optica 6, 1246 (2019).

▎原理与背景

太空光频梳应用1：寻找系外行星的精密径向速度测量

· 原理：当系外行星围绕恒星运行时，会使恒星产生微小的径向速度变化。通过测量恒星光谱中吸收线的多普勒频移，可以推断出恒星的径向速度变化，从而发现系外行星。频率梳可以提供高精度的频率参考，用于精确测量恒星光谱中吸收线的频率变化，进而提高径向速度测量的精度。

· 优势：传统的径向速度测量方法精度有限，难以探测到质量较小或距离较远的系外行星。而频率梳技术的高频率稳定性和精度，能够使径向速度测量精度达到厘米每秒甚至更高，从而大大提高了发现系外行星的概率和准确性。

太空光频梳应用2：

纳米级距离测量以支持卫星的超精密编队飞行或探测重力异常

· 原理：基于外差双光学频率梳的多波长干涉测距方法，利用单个频率梳齿进行最高精度的距离测量，由各级合成波长进行测程扩展。通过测量光在卫星之间传播的时间延迟，结合频率梳的高精度频率测量能力，实现纳米级的距离测量。

· 优势：在卫星超精密编队飞行中，纳米级的距离测量精度可以确保卫星之间的相对位置精确控制，从而实现更复杂的空间任务，如合成孔径雷达干涉测量等。在探测重力异常方面，精确的距离测量可以帮助监测地球重力场的微小变化，对于研究地球内部结构、海洋环流等具有重要意义。

太空光频梳应用3：飞秒时间传递以同步毫米波天文望远镜阵列

· 原理：利用光频梳在光频段与无线电射频段之间的桥梁作用，将光频梳产生的飞秒脉冲序列通过光纤或自由空间传输到各个望远镜站点，作为时间同步的基准信号。通过精确控制飞秒脉冲的传输时间和频率，实现毫米波天文望远镜阵列中各个望远镜之间的高精度时间同步。

·优势：毫米波天文观测需要多个望远镜协同工作，形成干涉阵列以提高观测分辨率。高精度的时间同步是保证干涉测量精度的关键，频率梳的飞秒时间传递技术可以将时间同步精度提高到飞秒量级，从而大大提高了毫米波天文望远镜阵列的观测性能。

太空光频梳应用4：对温室气体的灵敏检测

Principle: Frequency comb spectroscopy technology can generate broadband, high-resolution spectra covering the absorption spectral range of greenhouse gases. When light passes through a sample containing greenhouse gases, the gases absorb light at specific frequencies, resulting in absorption peaks in the spectrum. By measuring the positions and intensities of these absorption peaks, and utilizing the high-precision frequency measurement capability of the frequency comb, the types and concentrations of greenhouse gases can be accurately determined.

Advantages: Traditional greenhouse gas detection methods often suffer from low accuracy, slow response, and inability to monitor in real-time. Frequency comb spectroscopy technology offers high sensitivity, high resolution, and rapid response, enabling real-time, high-precision detection of greenhouse gases, providing strong technical support for climate change research and environmental monitoring.

Space Optical Frequency Comb Application 5: As a high-fidelity frequency divider crucial to optical clocks

Principle: Optical clocks are usually based on optical transitions of atoms or ions, with very high frequencies that are difficult to measure and apply directly. The frequency comb can act as a high-fidelity frequency divider, precisely dividing the optical frequency down to the microwave range, allowing comparison and calibration with traditional microwave atomic clocks, thereby achieving precise measurement and control of optical clocks.

Advantages: Optical clocks have higher frequency stability and accuracy than microwave atomic clocks and represent the future direction of time standards. As a key component of optical clocks, the frequency comb's high-fidelity frequency division capability ensures the full performance of optical clocks, providing technical assurance for achieving ultra-high precision time measurement and synchronization.